mt_monashmedfandomcom-20200213-history
L0301P72 - Principles of Organisation of Peripheral Spinal Nerves
Divisions of the Peripheral Nervous System Consists of: *12 cranial nerves and 31 spinal nerves Fibres: *somatic **innervates muscles, joints, skin (limbs and body wall) **motor - skeletal muscle **sensory ***afferent - goes towards the spine ***efferent - goes away from the spine **voluntary and conscious *visceral **efferent ***smooth and cardiac muscle, glands **afferent ***sensations from viscera (internal organs in the abdomen) Neurons *neurons interconnect with other neurons and travel in spinal nerves *many types: *axons take signals away from the cell body *dendrites send signals towards the cell body Typical Spinal Nerve *dorsal root stems out from the back of the spinal cord and ventral root from the front *roots meet just beyond the dorsal root ganglion to form the start of the spinal cord *dorsal ramus innervates muscles and skin on either side of the backbone *ventral ramus innervates trunk and limbs **splits into lateral cutaneous and anterior cutaneous nerve to innervate the skin *sympathetic rami consists of white and grey **i.e. white ramus communicans and great ramus communicans Peripheral Nerve Structure *made up by axons that travel in bundles with supporting elements *functional types (afferent, efferent, motor) cannot be distinguished from one another within the nerve Spinal Nerve Pairs *8 cervical - 7 vertebrae *12 thoracic - 12 vertebrae *5 lumbar - 5 vertebrae *5 sacral - 5 vertebrae *1 or 2 coccygeal *spinal cord has two enlargements: **cervical - innervate the upper limbs **lumbar - innervate the lower limbs Organs and the Fibres Synapse *nerve fibre depolarises = muscle contracts *the whole nerve does not depolarise as there are many fibres with different functions Reflex *sensory receptors in the skin detect stimulus *afferent neurons transmit the signal via the dorsal root ganglion *afferent neuron synapses with an interneuron in the spine *interneuron synapses with motor neurons of the ventral horn which signals for the muscle to contract Plexus *a network of intersecting nerves *composed of afferent and efferent fibres that arise from the merging of anterior rami Skin Sensation Modalities *complex but there are 5 main ones: **fine (discriminative) touch **vibration **light touch **temperature - warm/cold **pain *mixtures can be experienced Functional Types 1. Somatic Efferent *general motor *cell bodies in ventral grey horn of spine *synapse directly on striated muscle via motor end plate 2. Somatic Afferent *general sensory *cell bodies in dorsal root ganglia **pseudounipolar structure - axon splits into two branches; one to the periphery and other to the spinal cord *exteroceptors **mechanoreceptors = touch, pressure **thermoreceptors = temperature **nocireceptors = pain *proprioceptors **mechanoreceptors = joint position, vibration, skeletal muscle stretch **nocireceptors = deep pain Muscle Innervation *have both a sensory and motor supply *sensory - stretch receptors **tell the brain ‘how’ contracted the muscle is to allow for tendon reflex **via muscle spindles (intrafusal fibres) **numbers vary in different muscles ***abundant in antigravity muscles (back), neck muscles, small muscles of hand *joints have proprioceptors **detect position of limbs in space Entrapments *peripheral nerves can become compressed *caused by: **injury **inflammation **degenerative conditions **repetitive activity **iatrogenic - caused by medical treatment or examination **anatomical *e.g. carpal tunnel, tarsal tunnel syndrome Autonomic Nervous System *continuously controls viscera without relying on conscious awareness Subdivisions *sympathetic **fight or flight **thoracolumbar outflow **short preganglionic, long postganglionic *parasympathetic **rest and digest **craniosacral outflow **long preganglionic, short postganglionic Principles Sympathetics and parasympathetics have: *complementary actions **e.g. parasympathetic erection followed by sympathetic ejaculation *independent actions **a few viscera and almost all vessels receive sympathetics only **no parasympathetics in limbs *opposite actions **e.g. on heart rate and pupil size **as many organs receive supply from both Sympathetic Nervous System *sympathetic chain runs vertically on either side of the spinal cord *second order neurons stem form the sympathetic chain to the periphery via: **spinal nerves **cranial nerves **splanchnic nerves ***may also contain many first order neurons that did not synapse in sympathetic ganglia **blood vessels